Wed, January 10, 2018 Turnagain Pass Avalanche Forecast

Wed, January 10th, 2018

Alpine

Above 2,500'

2 - Moderate

Treeline

1,000'-2,500'

2 - Moderate

Below Treeline

Below 1,000'

1 - Low

0 - No Rating

1 - Low

2 - Moderate

3 - Considerable

4 - High

5 - Extreme


Travel Advice	Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.	Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.	Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding, and conservative decision-making essential.	Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.	Extraordinarily dangerous avalanche conditions. Avoid all avalanche terrain.
Likelihood of Avalanches	Natural and human-triggered avalanches unlikely.	Natural avalanches unlikely; human-triggered avalanches possible.	Natural avalanches possible; human-triggered avalanches likely.	Natural avalanches likely; human-triggered avalanches very likely.	Natural and human-triggered avalanches certain.
Avalanche Size and Distribution	Small avalanches in isolated areas or extreme terrain.	Small avalanches in specific areas; or large avalanches in isolated areas.	Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas.	Large avalanches in many areas; or very large avalanches in specific areas.	Very large avalanches in many areas.

Avalanche Problem 1

Persistent Slabs
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

Size

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.

Likelihood of Avalanches

Terms such as "unlikely", "likely", and "certain" are used to define the scale, with the chance of triggering or observing avalanches increasing as we move up the scale. For our purposes, "Unlikely" means that few avalanches could be triggered in avalanche terrain and natural avalanches are not expected. "Certain" means that humans will be able to trigger avalanches on many slopes, and natural avalanches are expected.

Size of Avalanches

Avalanche size is defined by the largest potential avalanche, or expected range of sizes related to the problem in question. Assigned size is a qualitative estimate based on the destructive classification system and requires specialists to estimate the harm avalanches may cause to hypothetical objects located in the avalanche track (AAA 2016, CAA 2014). Under this schema, "Small" avalanches are not large enough to bury humans and are relatively harmless unless they carry people over cliffs or through trees or rocks. Moving up the scale, avalanches become "Large" enough to bury, injure, or kill people. "Very Large" avalanches may bury or destroy vehicles or houses, and "Historic" avalanches are massive events capable of altering the landscape.

Signal Word	Size (D scale)	Simple Descriptor
Small	1	Unlikely to bury a person
Large	2	Can bury a person
Very Large	3	Can destroy a house
Historic	4 & 5	Can destroy part or all of a village

More info at Avalanche.org

As we move further from our last loading event (New Year’s Storm) and cold temperatures degrade the upper layers of the snowpack, the chances of triggering a slab avalanche are decreasing. However, it is not something that should totally be ruled out yet. We know that the snow from the storm is sitting on either buried surface hoar, buried near-surface facets or in thin snowpack areas, facets near the ground. This means there is poor snowpack structure and a persistent slab set-up above 2000′. Snowpack tests over the past few days have shown variable results but point to the continued potential for triggering an avalanche.

For those riders and skiers headed out today:

Triggering a slab is most likely in the upper elevations above 2,000′, where NO crusts exist in the top foot of the snowpack.
Several tracks may be on the slope before a slab releases and no signs saying ‘the slope is unstable’ are likely to be present.
Larger slopes, 35 degrees and steeper are more suspect and those with rocky features.
Practice safe travel habits: expose only one person at a time, have an escape route planned, watch your buddies closely and view slopes as avalanche paths. If the snow does slide, where will it go? Avoid terrain traps.

Snow pit on Sunburst January 8th and a snow pit from Seattle Ridge on January 7th. These illustrate the weak layers of snow in the snowpack under a slab, that in both spots the snowpack is still reactive and that an avalanche may be triggered.

Avalanche Problem 2

Deep Persistent Slabs
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

Size

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.

Likelihood of Avalanches

Size of Avalanches

Signal Word	Size (D scale)	Simple Descriptor
Small	1	Unlikely to bury a person
Large	2	Can bury a person
Very Large	3	Can destroy a house
Historic	4 & 5	Can destroy part or all of a village

More info at Avalanche.org

Deep Persistent Slabs have been in the forecast every day starting December 14th. Because a poor snowpack structure still exists at high elevations above 3,000’, human triggered large and dangerous, deep slab avalanches are still possible. Weak sugary snow (basal facets) near the ground is creating a low probability/high consequence avalanche problem. Likely trigger spots will be in thinner areas of the snowpack that are connected to large, loaded slopes. Signs of instability will not likely be present and there may be tracks on the slope. The possibility of a deep slab avalanche should still be part of your decision-making before committing to big terrain in the Alpine.

Additional Concern

Dry Loose

Dry Loose

Dry Loose avalanches are the release of dry unconsolidated snow and typically occur within layers of soft snow near the surface of the snowpack. These avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs.

More info at Avalanche.org

Cold temperatures are continuing to weaken the surface snow. Observers in the Turnagain area yesterday noted this. Sluffs (loose snow avalanches) are getting larger and could catch you off guard in steep terrain.

Weather

Wed, January 10th, 2018

Yesterday was mostly clear above the valley fog. The fog did not move out contrary to the morning forecast. Temperatures were in the teens to single digits with a minor inversion. Winds were easterly 5-15 with gusts in the high teens.

Today will be partly to mostly cloudy and there is a chance of snow showers this afternoon. Temperatures will warm into the high teens, low 20s. Winds will be easterly 5-15 with gusts into the 20s. Snow showers will continue overnight with a chance of 1-2″ of accumulation. Easterly winds will pick up this evening gusting into the 30s and temperatures will be in the teens to low 20s.

The pattern for the next few days sets up with warmer air and a series of storms on track to impact the area. Stay tuned for timing, precipitation amounts and temperatures.

PRECIPITATION 24-hour data (6am – 6am)

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	18	0	0	43
Summit Lake (1400′)	7	0	0	15
Alyeska Mid (1700′)	14	0	0	35

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	11	E	8	19
Seattle Ridge (2400′)	15	SE	12	16

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
05/13/24	Turnagain	Observation: Eddie’s, Sunburst, Seattle, Cornbiscuit, Pete’s South	H Thamm
05/13/24	Turnagain	Observation: Turnagain Pass non-motorized side	Amy Holman
05/12/24	Turnagain	Observation: Warm up Bowl	Tony Naciuk
05/07/24	Turnagain	Observation: Turnagain Pass Wet Slabs	A S
04/29/24	Turnagain	Avalanche: Turnagain aerial obs	Tully Hamer
04/27/24	Turnagain	Observation: Johnson Pass	Noah Mery
04/23/24	Turnagain	Observation: Turnagain Sunny Side	Travis SMITH
04/21/24	Turnagain	Observation: Bertha Creek	Anonymous
04/20/24	Turnagain	Avalanche: Spokane Creek	Schauer/ Mailly Forecaster
04/16/24	Turnagain	Observation: Cornbiscuit	Krueger / Matthys Forecaster

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